In many medical procedures, medical optical appliances are used to examine or treat a patient. These appliances are often mounted on an articulated holding device, such that they can be positioned at different positions in the work space and, if appropriate, can be oriented there in a desired direction. In operation, it is often necessary to vary the position and optionally the orientation of the medical optical appliance, for example in order to modify a viewing angle of the operating site by a surgical microscope or to modify a beam direction of a treatment laser. There is therefore a frequent alternation between stationary phases, in which the medical optical appliance is held in a defined position in space, and movement phases, in which the medical optical appliance is transferred from one position to another position.
In a movement phase, the medical optical appliance is intended to be able to be transferred as smoothly as possible by a user, that is to say by application of minimal physical force, from a starting position to an end position. It is known from the prior art to provide the holding device with actuators on the joints, which actuators assist or indeed actually permit a movement of the stand.
DE 10 2004 063 606 A1 discloses a medical apparatus comprising a medical optical appliance in the form of a surgical microscope which is mounted on a holding device. For force-free movement of the surgical microscope, the medical apparatus has electric motors on the joints of the holding device. Angle settings in the rotary joints are detected by position sensors. Current control curves are stored in an electronic memory, and current values can be determined from these as a function of the detected angle settings. A current value is determined from the detected angle settings with the aid of the current control curves, with which current value the electric motors are powered such that a defined torque is generated in the electric motors. In this way, a static load torque can be compensated by the counter-torque generated in the electric motors, such that the surgical microscope can be moved by a user without any apparent force. However, the medical apparatus according to DE 10 2004 063 606 A1 has disadvantages in the unguided state when a user is not actively moving the medical apparatus or holding it at a position in space. In this case, drift can occur, i.e. a creeping movement of the medical optical appliance away from a set position. A further disadvantage of the medical apparatus is that (inadvertent) bumping against the holding device or the surgical microscope can result in an uncontrolled movement, which is something to be avoided in a surgical situation.
To avoid uncontrolled movements, it is known from the prior art to provide holding devices with brakes that can be activated by a user in order to suppress some or all of the degrees of freedom of movement of the holding device, such that a position and/or orientation of the medical optical appliance in space is maintained. Holding devices with brakes are disclosed, for example, in the documents DE 101 44 033 A1, DE 202 18 693 U1 and DE 20 2007 003 822 U1. However, a disadvantage of using brakes is that they increase the complexity of the overall system, have a flexibility that promotes a tendency toward vibration, and generally have to be actively opened and closed a number of times during an operation, which impedes the progress of the operation